LDH separator and secondary zinc battery

ABSTRACT

Provided is a layered double hydroxide (LDH) separator including a porous substrate made of a polymeric material; and a hydroxide-ion conductive layered compound being a LDH and/or a LDH-like compound with which pores of the porous substrate are plugged. The LDH separator has a mean porosity of 0.03% to less than 1.0%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 16/903,673 filed Jun. 17, 2020, which is acontinuation application of PCT/JP2018/045885 filed Dec. 13, 2018, whichclaims priority to Japanese Patent Application No. 2017-241988 filedDec. 18, 2017 and Japanese Patent Application No. 2018-114670 filed Jun.15, 2018, the entire contents all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a LDH separator and a secondary zincbattery.

2. Description of the Related Art

In secondary zinc batteries, such as secondary nickel-zinc batteries andsecondary air-zinc batteries, it is known that metallic zinc dendritesprecipitate on negative electrodes during a charge mode, penetratethrough voids in separators composed of, for example, non-woven fabrics,and reach positive electrodes, resulting in short circuit. The shortcircuit caused by such zinc dendrites occurs during repeatedcharge/discharge operations, leading to a reduction in service lives ofthe secondary zinc batteries.

In order to solve such a problem, secondary zinc batteries have beenproposed that include layered double hydroxide (LDH) separators thatselectively permeate hydroxide ions while blocking the penetration ofzinc dendrites. For example, PTL 1 (WO2013/118561) discloses a secondarynickel-zinc battery including a LDH separator disposed between apositive electrode and a negative electrode. PTL 2 (WO2016/076047)discloses a separator structure including a LDH separator that is fit inor joined to a resin frame and is dense enough to restrict permeation ofgas and/or water. PTL 2 also discloses that the LDH separator may be acomposite with a porous substrate. In addition, PTL 3 (WO 2016/067884)discloses various methods for forming a dense LDH membrane on thesurface of a porous substrate to give a composite material (a LDHseparator). These methods include the steps of: uniformly bonding aninitiating material capable of giving origins of crystal growth of LDHto the porous substrate; and then subjecting the porous substrate tohydrothermal treatment in an aqueous raw material solution to form adense LDH membrane on the surface of the porous substrate.

CITATION LIST Patent Literature

PTL1: WO2013/118561

PTL2: WO2016/076047

PTL3: WO2016/067884

SUMMARY OF THE INVENTION

In the case that secondary zinc batteries, for example, nickel-zincbatteries, are constructed with a LDH separator as described above, theproblem such as short circuit caused by zinc dendrites can beeffectively prevented to some extent. However, a further improvement isdesired for a preventive effect of the short circuit caused by thedendrites.

The present inventors have discovered that by plugging pores of a porouspolymeric substrate with LDH and highly densifying the substrate so asto have a mean porosity of 0.03% to less than 1.0%, it is possible toprovide a LDH separator that can more effectively prevent shortcircuiting caused by zinc dendrites.

Accordingly, an object of the present invention is to provide a LDHseparator capable of more effectively restraining the short circuitcaused by zinc dendrites.

According to an embodiment of the present invention, there is provided alayered double hydroxide (LDH) separator comprising a porous substratemade of a polymeric material; and a hydroxide-ion conductive layeredcompound being a LDH and/or a LDH-like compound with which pores of theporous substrate are plugged, wherein the LDH separator has a meanporosity of 0.03% to less than 1.0%.

According to another embodiment of the present invention, there isprovided a secondary zinc battery comprising the LDH separator.

According to another embodiment of the present invention, there isprovided a solid-state alkaline fuel cell comprising the LDH separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual schematic cross-sectional view illustrating a LDHseparator of the present invention.

FIG. 2 is a schematic cross-sectional view of a measurement device usedin the dendrite short-circuiting test in Examples 1 to 8.

FIG. 3A is a conceptual view illustrating an example system formeasuring helium permeability used in Examples 1 to 8.

FIG. 3B is a schematic cross-sectional view of a sample holder and itsperipheral configuration used in the measurement system shown in FIG.3A.

FIG. 4 is a schematic cross-sectional view illustrating anelectrochemical measurement system used in Examples 1 to 8.

FIG. 5 is a cross-sectional field-emission scanning electron microscopic(FE-SEM) image of a LDH separator produced in Example 4. In the drawing,the gray areas correspond to the porous polymeric substrate, the whiteareas LDH, and the black areas remaining pores.

FIG. 6 is a cross-sectional FE-SEM image of another LDH separatorproduced in Example 8 (Comparative Example). In the drawing, the grayareas correspond to the porous polymeric substrate, the white areas theLDH, and the black areas remaining pores.

DETAILED DESCRIPTION OF THE INVENTION LDH Separator

As illustrated in a schematic cross-sectional view in FIG. 1 , a LDHseparator 10 of the present invention includes a porous substrate 12 anda hydroxide-ion conductive layered compound 14 that is a layered doublehydroxide (LDH) and/or a LDH-like compound. Throughout thespecification, “LDH separator” is defined as a separator that includes aLDH and/or a LDH-like compound (collectively referred to as a“hydroxide-ion conductive layered compound”) and can selectively permithydroxide ions to migrate solely by means of hydroxide ion conductivityof the hydroxide-ion conductive layered compound. The term “LDH-likecompound” as used herein means a hydroxide and/or an oxide with alayered crystal structure analogous to LDH, which may not be called LDH,and can be said to be an equivalent of LDH. However, as a broaddefinition, “LDH” can be interpreted to include not only LDH but alsoLDH-like compounds. Although individual regions of the hydroxide-ionconductive layered compound 14 are seemed to be discontinuous betweenthe top face and bottom face of the LDH separator 10 in atwo-dimensional cross-section of the LDH separator in FIG. 1 , theregions of the hydroxide-ion conductive layered compound 14 arecontinuous in the three-dimensional geometry including the depth betweenthe top face and bottom face of the LDH separator 10, which ensures thehydroxide ion conductivity of the LDH separator 10. The porous substrate12 is made of a polymeric material. Pores of the porous substrate 12 areplugged with the hydroxide-ion conductive layered compound 14. In thisregard, the pores of the porous substrate 12 are not completely pluggedwith the hydroxide-ion conductive layered compound 14 and a small numberof pores P remains. Due to such remaining pores P, the LDH separator 10has a mean porosity of 0.03% to less than 1.0%. In other words, the LDHseparator 10 of the present invention has a significantly reduced numberof remaining pores P (pores not plugged with the hydroxide-ionconductive layered compound). A mean porosity of 0.03% to less than 1.0%indicates that the pores of the porous substrate 12 are sufficientlyplugged with the hydroxide-ion conductive layered compound and that theporous substrate 12 has significantly high denseness. In this way, thepores of the porous polymeric substrate 12 are plugged with thehydroxide-ion conductive layered compound and the porous substrate issignificantly densified so as to have a mean porosity of 0.03% to lessthan 1.0%. The LDH separator 10 can be thereby provided that can moreeffectively prevent the short circuiting caused by zinc dendrites. In atraditional separator, penetration of zinc dendrites is assumed to occurby the following mechanism: (i) Zinc dendrites intrude into voids ordefects contained in the separator; (ii) the dendrites grow or propagatewhile expanding the voids or defects in the separator; and (iii) thedendrites finally penetrate the separator. In contrast, the pores of theporous substrate 12 are sufficiently plugged with the hydroxide-ionconductive layered compound and thus the LDH separator 10 of the presentinvention is densified into a mean porosity of 0.03% to less than 1.0%that does not allow the intrusion or propagation of the zinc dendrites.Hence, the short circuiting caused by the zinc dendrites can be moreeffectively prevented.

The LDH separator 10 of the present invention has excellent flexibilityand strength, as well as a desired ion conductivity based on thehydroxide ion conductivity of the hydroxide-ion conductive layeredcompound 14. The flexibility and strength are caused by those of theporous polymeric substrate 12 itself of the LDH separator 10. In otherwords, the LDH separator 10 is densified in such a manner that the poresof the porous polymeric substrate 12 are sufficiently plugged with thehydroxide-ion conductive layered compound 14, and the porous polymericsubstrate 12 and the hydroxide-ion conductive layered compound 14 arehighly integrated into a superior composite material, thereby highrigidity and low ductility caused by the hydroxide-ion conductivelayered compound 14, which is ceramic material, can be balanced with orreduced by high flexibility and high strength of the porous polymericsubstrate 12.

The LDH separator 10 has a mean porosity of 0.03% to less than 1.0%,preferably 0.05% to 0.95%, more preferably 0.05% to 0.9%, furtherpreferably 0.05 to 0.8%, most preferably 0.05 to 0.5%. The LDH separator10 having a mean porosity within such a range exhibits significantlyhigh denseness, where the pores of porous substrate 12 are sufficientlyplugged with the hydroxide-ion conductive layered compound 14. Thus, theshort circuiting caused by zinc dendrites can be more effectivelyprevented. A significantly high ion conductivity of the LDH separator 10can be also achieved. Thus, the LDH separator 10 can conduct asufficiently high amount of hydroxide ions. The mean porosity can bedetermined by a) polishing a cross-sectional face of the LDH separatorwith a cross-sectional polisher (CP), b) capturing two fields of imagesof a functional layer at a magnification of 50,000 folds with afield-emission scanning electron microscope (FE-SEM), c) calculatingporosities of the two fields with image inspection software (forexample, HDevelop available from MVTec Software) based on the data ofthe captured cross-sectional images, and d) averaging the calculatedporosities.

The LDH separator 10 has an ionic conductivity of preferably 0.1 mS/cmor more, more preferably 1.0 mS/cm or more, further preferably 1.5 mS/cmor more, particularly preferably 2.0 mS/cm or more. Such a range allowsthe LDH separator to fully function as a separator having hydroxideionic conductivity. Since a higher ionic conductivity is preferred, theLDH separator may have any upper limit of ionic conductivity, forexample, 10 mS/cm. The ionic conductivity is calculated from theresistance, the thickness and the area of the LDH separator. Theresistance of the LDH separator 10 is measured within a frequency rangeof 1 MHz to 0.1 Hz and under an applied voltage of 10 mV using anelectrochemical measurement system (potentio-galvanostat frequencyresponsive analyzer) for the LDH separator 10 immersed in an aqueous KOHsolution of a predetermined concentration (for example, 5.4 M), and theintercept across the real axis can be determined to be the resistance ofthe LDH separator.

The LDH separator 10 includes a hydroxide-ion conductive layeredcompound 14, and can isolate a positive electrode plate from a negativeelectrode plate and ensures hydroxide ionic conductivity therebetween ina secondary zinc battery. The LDH separator 10 functions as a hydroxideionic conductive separator. Preferred LDH separator 10 hasgas-impermeability and/or water-impermeability. In other words, the LDHseparator 10 is preferably densified to an extent that exhibitsgas-impermeability and/or water-impermeability. The phrase “havinggas-impermeability” throughout the specification indicates that nobubbling of helium gas is observed at one side of a sample when heliumgas is brought into contact with the other side in water at adifferential pressure of 0.5 atm as described in PTLs 2 and 3. Inaddition, the phrase “having water-impermeability” throughout thespecification indicates that water in contact with one side of thesample does not permeate to the other side as described in PTLs 2 and 3.As a result, the LDH separator 10 having gas-impermeability and/orwater-impermeability indicates having high density to an extent that nogas or no water permeates, and not being a porous membrane or any otherporous material that has gas-permeability or water-permeability.Accordingly, the LDH separator 10 can selectively permeate onlyhydroxide ions due to its hydroxide ionic conductivity, and can serve asa battery separator. The LDH separator 10 thereby has a physicalconfiguration that prevents penetration of zinc dendrites generatedduring a charge mode through the separator, resulting in prevention ofshort circuit between positive and negative electrodes. Since the LDHseparator 10 has hydroxide ionic conductivity, the ionic conductivityallows a necessary amount of hydroxide ions to efficiently move betweenthe positive electrode plate and the negative electrode plate, andthereby charge/discharge reaction can be achieved on the positiveelectrode plate and the negative electrode plate.

The LDH separator 10 preferably has a helium permeability per unit areaof 3.0 cm/min·atm or less, more preferably 2.0 cm/min·atm or less,further preferably 1.0 cm/min·atm or less. A separator having a heliumpermeability of 3.0 cm/min·atm or less can remarkably restrain thepermeation of Zn (typically, the permeation of zinc ions or zincateions) in the electrolytic solution. Thus, it is conceivable in principlethat the separator of the present embodiment can effectively restrainthe growth of zinc dendrites when used in secondary zinc batteriesbecause Zn permeation is significantly suppressed. The heliumpermeability is measured through the steps of: supplying helium gas toone side of the separator to allow the helium gas to permeate into theseparator; and calculating the helium permeability to evaluate thedensity of the hydroxide ion conductive separator. The heliumpermeability is calculated from the expression of F/(P×S) where F is thevolume of permeated helium gas per unit time, P is the differentialpressure applied to the separator when helium gas permeates through, andS is the area of the membrane through which helium gas permeates.Evaluation of the permeability of helium gas in this manner canextremely precisely determine the density. As a result, a high degree ofdensity that does not permeate as much as possible (or permeate only atrace amount) substances other than hydroxide ions (in particular, zincthat causes deposition of dendritic zinc) can be effectively evaluated.Helium gas is suitable for this evaluation because the helium gas hasthe smallest constitutional unit among various atoms or molecules whichcan constitute the gas and its reactivity is extremely low. That is,helium does not form a molecule, and helium gas is present in the atomicform. In this respect, since hydrogen gas is present in the molecularform (H₂), atomic helium is smaller than molecular H₂ in a gaseousstate. Basically, H₂ gas is combustible and dangerous. By using thehelium gas permeability defined by the above expression as an index, thedensity can be precisely and readily evaluated regardless of differencesin sample size and measurement condition. Thus, whether the separatorhas sufficiently high density suitable for separators of secondary zincbatteries can be evaluated readily, safely and effectively. The heliumpermeability can be preferably measured in accordance with the procedureshown in Evaluation 5 in Examples described later.

In the LDH separator 10 of the present invention, the pores in theporous substrate 12 are plugged with the hydroxide-ion conductivelayered compound 14 that is the LDH and/or the LDH-like compound. As isgenerally known, the LDH is composed of a plurality of basic hydroxidelayers and intermediate layers interposed between these basic hydroxidelayers. The basic hydroxide layers are each mainly composed of metallicelements (typically metallic ions) and OH groups. The intermediatelayers of the LDH are composed of anions and H₂O. The anions aremonovalent or multivalent anions, preferably monovalent or divalentions. The anions in the LDH preferably include OH⁻ and/or CO₃ ²⁻. TheLDH has high ionic conductivity based on its inherent properties.

In general, the LDH is known to typically have the fundamental formula:M²⁺ _(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/n)·mH₂O (wherein, M²⁺ is a divalentcation, M³⁺ is a trivalent cation, A^(n−) is n-valent anion, n is aninteger of 1 or more, x is 0.1 to 0.4, and m is 0 or more). In the abovefundamental formula, M²⁺ may be any divalent cation, and includes,preferably Mg²⁺, Ca²⁺ and Zn²⁺, more preferably Mg²⁺. M³⁺ may be anytrivalent cation, and includes, preferably Al³⁺ and Cr³⁺, morepreferably Al³⁺. A^(n−) may be any anion, and preferably includes OH⁻and CO₃ ²⁻. Accordingly, it is preferred that M²⁺ includes Mg²⁺, M³⁺includes Al³⁺, and An⁻ includes OH⁻ and/or CO₃ ²⁻ in the abovefundamental formula. n is an integer of 1 or more, preferably 1 or 2. xis 0.1 to 0.4, preferably 0.2 to 0.35. m is an any molar number ofwater, and is a real number of 0 or more, typically a real number ofmore than 0 or 1 or more. However, the above fundamental formula merelyrepresents “a fundamental composition” typically illustrated for theLDH, and constituent ions can be appropriately replaced. For example, inthe above fundamental formula, part or the whole of M³⁺ may be replacedwith a tetravalent or higher valence cation, where the coefficient x/nof the anion A^(n−) may be appropriately varied in the above fundamentalformula.

For example, the basic hydroxide layers of LDH may be composed of Ni,Ti, OH groups and optional incidental impurities. The intermediatelayers of LDH are composed of anions and H₂O as described above.Although the alternately stacked structure itself of basic hydroxidelayers and intermediate layers is basically the same as the commonlyknown alternately stacked structure of LDH, the LDH of the embodiment,which is composed of the basic hydroxide layers mainly having Ni, Ti andOH groups of LDH, can exhibit high alkaline resistance. Although thereason is not clear, it is believed that no element (for example, Al)readily dissolved in an alkaline solution is intentionally added to theLDH of the embodiment. Nevertheless, the LDH of the embodiment can alsoexhibit high ionic conductivity suitable for separators for alkalinesecondary batteries. Ni in the LDH can have the form of nickel ions.Although nickel ions in the LDH are typically believed to be Ni²⁺, theymay be present in any other valence, for example, Ni³⁺. Ti in the LDHcan have the form of titanium ions. Although titanium ions in the LDHare typically believed to be Ti⁴⁺, they may be present in any othervalence, for example, Ti³⁺. Each of the incidental impurities is anyelement which may be inevitably mixed in a manufacturing process, and itmay be mixed into the LDH from, for example, a raw material or asubstrate. As described above, it is impractical or impossible tostrictly specify the LDH with a general formula since valences of Ni andTi are not necessarily confirmed. Assuming that the basic hydroxidelayers are mainly composed of Ni²⁺, Ti⁴⁺ and OH groups, the fundamentalcomposition of the corresponding LDH can be represented by the generalformula: Ni²⁺ _(1−x)Ti⁴⁺ _(x)(OH)₂A^(n−) _(2x/n)·mH₂O, wherein A^(n−) isan n-valent anion, n is an integer of 1 or more, preferably 1 or 2, x isabove 0 to below 1, preferably 0.01 to 0.5, and m is a real number of 0or more, typically a real number above 0 or 1 or more. However, itshould be understood that the general formula indicates merely the“fundamental composition”, and it may be replaced with other elements orions (including elements with other valences of the same element, orelements or ions that may be unavoidably mixed in the manufacturingprocess) to such an extent that the elements such as Ni²⁺, and Ti⁴⁺ donot impair the basic properties of LDH.

Alternatively, the basic hydroxide layers of LDH comprise Ni, Al, Ti andOH groups. The intermediate layers are composed of anions and H₂O asdescribed above. Although the alternately stacked structure itself ofbasic hydroxide layers and intermediate layers is basically the same asthe generally known alternately stacked structure of LDH, the LDH of theembodiment, in which the basic hydroxide layers of the LDH are composedof predetermined elements and/or ions including Ni, Al, Ti and OHgroups, can exhibit high alkaline resistance. Although the reason is notclear, it is believed that Al, which has been considered to be readilydissolved in an alkaline solution, is hard to elute into the alkalinesolution due to some interaction with Ni and Ti. Nevertheless, the LDHof the embodiment can also exhibit high ionic conductivity suitable forseparators for alkaline secondary batteries. Ni in the LDH can have theform of nickel ions. Although nickel ions in the LDH are typicallybelieved to be Ni²⁺, they may be present in any other valence, forexample, Ni³⁺. Al in the LDH can have the form of aluminum ions.Although aluminum ions in the LDH are typically believed to be A^(n−),they may be present in any other valence. Ti in the LDH can have theform of titanium ions. Although titanium ions in the LDH are typicallybelieved to be Ti⁴⁺, they may be present in any other valence, forexample, Ti³⁺. The basic hydroxide layers may contain other elements orions as long as they contain Ni, Al, Ti and OH groups. However, thebasic hydroxide layers preferably contain Ni, Al, Ti and OH groups asmain constituent elements. That is, it is preferred that the basichydroxide layers are mainly composed of Ni, Al, Ti and OH groups.Accordingly, the basic hydroxide layers are typically composed of Ni,Al, Ti, OH groups and optional incidental impurities. Each of theincidental impurities is any element which may be inevitably mixed in amanufacturing process, and it may be mixed into the LDH from, forexample, a raw material or a substrate. As described above, it isimpractical or impossible to strictly specify the LDH with a generalformula since valences of Ni, Al and Ti are not necessarily confirmed.Assuming that the basic hydroxide layers are mainly composed of Ni²⁺,Al³⁺, Ti⁴⁺ and OH groups, the fundamental composition of thecorresponding LDH can be represented by the general formula: Ni²⁺_(1−x−y)Al³⁺ _(x)Ti⁴⁺ _(y)(OH)₂A^(n−) _((x+2y)/n)·mH₂O, wherein A^(n−)is an n-valent anion, n is an integer of 1 or more, preferably 1 or 2, xis above 0 to below 1, preferably 0.01 to 0.5, y is above 0 to below 1,preferably 0.01 to 0.5, x+y is above 0 to below 1, and m is a realnumber of 0 or more, typically a real number of above 0 or 1 or more.However, it should be understood that the general formula indicatesmerely the “fundamental composition”, and it may be replaced with otherelements or ions (including elements with other valences of the sameelement, or elements or ions that may be unavoidably mixed in themanufacturing process) to such an extent that the elements such as Ni²⁺,Al³⁺ and Ti⁴⁺ do not impair the basic properties of LDH.

The LDH-like compound is a hydroxide and/or an oxide with a layeredcrystal structure analogous to LDH, which may not be called LDH,preferably including Mg and one or more elements including at least Tiselected from the group consisting of Ti, Y, and Al. Thus, by using aLDH-like compound, which is a hydroxide and/or oxide with a layeredcrystal structure containing at least Mg and Ti, as a hydroxide-ionconductor instead of the conventional LDH, it is possible to provide aLDH separator with excellent alkali resistance which can moreeffectively suppress the short circuit caused by zinc dendrite.Accordingly, the preferred LDH-like compound is a hydroxide and/or oxidewith a layered crystal structure containing Mg and one or more elementsincluding at least Ti selected from the group consisting of Ti, Y andAl. Therefore, the typical LDH-like compound is a composite hydroxideand/or composite oxide of Mg, Ti, optionally Y and optionally Al, andespecially preferably a composite hydroxide and/or composite oxide ofMg, Ti, Y and Al. The above elements may be replaced by other elementsor ions to the extent that the fundamental properties of the LDH-likecompound are not impaired, but the LDH-like compound is preferably freeof Ni.

The LDH-like compounds can be identified by X-ray diffraction.Specifically, when X-ray diffraction is performed on the surface of theLDH separator, peaks derived from a LDH-like compound is typicallydetected in the range of 5°≤2θ≤10° and more typically in the range of7°≤2θ≤10°. As described above, LDH is a material having an alternatinglayered structure in which exchangeable anions and H₂O exist asintermediate layers between the stacked basic hydroxide layers. In thisregard, when LDH is measured by X-ray diffraction, a peak derived fromthe crystal structure of LDH (i.e., the (003) peak of LDH) is originallydetected at 2θ=11-12°. On the other hand, when the LDH-like compound ismeasured by X-ray diffraction, a peak is typically detected within theaforementioned range at a position shifted to a lower angle than that ofLDH. In addition, the interlayer distance of the layered crystalstructure can be determined by the Bragg's equation using 2θcorresponding to the peak derived from the LDH-like compound in X-raydiffraction. The interlayer distance of the layered crystal structureconstituting the LDH-like compound thus determined is typically 0.883 to1.8 nm, and more typically 0.883 to 1.3 nm.

The atomic ratio of Mg/(Mg+Ti+Y+Al) in the LDH-like compound, asdetermined by energy dispersive X-ray analysis (EDS), is 0.03 to 0.25,preferably 0.05 to 0.2. The atomic ratio of Ti/(Mg+Ti+Y+Al) in theLDH-like compound is preferably 0.40 to 0.97, more preferably 0.47 to0.94. In addition, the atomic ratio of Y/(Mg+Ti+Y+Al) in the LDH-likecompound is preferably 0 to 0.45, more preferably 0 to 0.37. The atomicratio of Al/(Mg+Ti+Y+Al) in the LDH-like compound is preferably 0 to0.05, more preferably 0 to 0.03. Within the above range, it is possibleto achieve even better alkali resistance and more effective suppressionof the short circuit caused by zinc dendrite (i.e., dendriteresistance). The LDH conventionally known for the LDH separator can berepresented by the basic composition of the general formula: M²⁺_(1−x)M³⁺ _(x)(OH)₂A^(n−) _(x/m)·mH₂O (where M²⁺ is a divalent cation,M³⁺ is a trivalent cation, A^(n−) is an n-valent anion, n is an integergreater than or equal to 1, x is 0.1 to 0.4, and m is 0 or greater). Incontrast, the above atomic ratios of the LDH-like compound generallydeviate from the general formula of LDH. Thus, the LDH-like compoundgenerally have different composition ratios (atomic ratios) thanconventional LDH. EDS analysis is preferably performed by using an EDSanalyzer (e.g., X-act, manufactured by Oxford Instruments), 1) capturingimages at an acceleration voltage of 20 kV and a magnification of 5,000times, 2) conducting a three-point analysis with an interval of about 5μm in the point analysis mode, 3) repeating the above 1) and 2) oncemore, and 4) calculating the average value of six points in total.

As described above, the LDH separator 10 comprises the hydroxide-ionconductive layered compound 14 and the porous substrate 12 (typicallyconsists of the porous substrate 12 and the hydroxide-ion conductivelayered compound 14), and the hydroxide-ion conductive layered compound14 plugs the pores in the porous substrate 12 such that the LDHseparator 10 exhibits hydroxide ionic conductivity andgas-impermeability (thus, so as to serve as a LDH separator exhibitinghydroxide ionic conductivity). In particular, the hydroxide-ionconductive layered compound 14 is preferably incorporated over theentire thickness of the porous substrate 12 composed of a polymericmaterial. The LDH separator has a thickness of preferably 3 to 80 μm,more preferably 3 to 60 μm, further preferably 3 to 40 μm.

The porous substrate 12 is composed of a polymeric material. Thepolymeric porous substrate 12 has the following advantages; (1) highflexibility (hard to crack even if thinned), (2) high porosity, (3) highconductivity (small thickness with high porosity), and (4) goodmanufacturability and handling ability. The polymeric porous substratehas a further advantage; (5) readily folding and sealing the LDHseparator including the porous substrate composed of the polymericmaterial based on the advantage (1): high flexibility. Preferredexamples of the polymeric material include polystyrene, poly(ethersulfone), polypropylene, epoxy resin, poly(phenylene sulfide),fluorocarbon resin (tetra-fluorinated resin such as PTFE), cellulose,nylon, polyethylene and any combination thereof. More preferred examplesinclude polystyrene, poly(ether sulfone), polypropylene, epoxy resin,poly(phenylene sulfide), fluorocarbon resin (tetra-fluorinated resinsuch as PTFE), nylon, polyethylene and any combination thereof from theviewpoint of a thermoplastic resin suitable for hot pressing. All thevarious preferred materials described above have alkali resistance to beresistant to the electrolytic solution of batteries. More preferredpolymeric materials are polyolefins, such as polypropylene andpolyethylene, most preferred are polypropylene and polyethylene from theviewpoint of excellent hot-water resistance, acid resistance and alkaliresistance, and low material cost. In case that the porous substrate iscomposed of the polymeric material, the hydroxide-ion conductive layeredcompound is particularly preferably embedded over the entire thicknessof the porous substrate (for example, most pores or substantially allpores inside the porous substrate are filled with the hydroxide-ionconductive layered compound). A polymeric microporous membranecommercially available can be preferably used as such a polymeric poroussubstrate.

Manufacturing Process

The LDH separator of the present invention is manufactured by the stepsof: (i) preparing a composite material containing a hydroxide-ionconductive layered compound (i.e., a crude LDH separator) according to aknown procedure using a polymeric porous substrate, and (ii) pressingthe composite material containing a hydroxide-ion conductive layeredcompound. The step of pressing may be performed by any procedure, suchas roll pressing, uniaxial pressure pressing, and CIP (cold isostaticpressing), preferably roll pressing. The step of pressing preferablyinvolves heating to soften the porous polymeric substrate and therebysufficiently plug the pores of the porous substrate with thehydroxide-ion conductive layered compound. For example, the heatingtemperature required for enough softening is preferably 60 to 200° C. inthe case that the polymer in the substrate is polypropylene orpolyethylene. The step of pressing, for example, roll pressing in such atemperature range can significantly reduce the mean porosity due toreductions in remaining pores of the LDH separator. As a result, the LDHseparator can be significantly densified and thus more effectivelyprevent the short circuiting caused by zinc dendrites. Appropriateadjustments of the gap between rollers and the roller temperature inroll pressing can control the shapes of the remaining pores, resultingin a LDH separator having desired denseness or a desired mean porosity.

The composite material containing the hydroxide-ion conductive layeredcompound (i.e., the crude LDH separator) before the step of pressing canbe produced by any process, preferably by appropriate modification ofvarious conditions in known methods (e.g., see PTLs 1 to 3) forproducing the functional layer and the composite material containing thehydroxide-ion conductive layered compound (that is, the LDH separator).For example, the functional layer and the composite material containingthe hydroxide-ion conductive layered compound (that is, the LDHseparator) can be produced by the Steps of: (1) providing a poroussubstrate; (2) applying a titania sol (for forming a LDH) or a mixed solof titania and yttrium and/or alumina (for forming a LDH-like compound)onto the porous substrate and then heating the sol to form atitania-containing layer (e.g., titanium oxide layer or analumina/titania layer); (3) immersing the porous substrate into anaqueous raw material solution containing nickel ions (Ni²⁺) or magnesiumions (Mg²⁺) and urea (or further containing yttrium ions (Y³⁺); and (4)hydrothermally treating the porous substrate in the aqueous raw materialsolution to form the functional layer containing the hydroxide-ionconductive layered compound on the porous substrate and/or in a poroussubstrate. In particular, in Step (2), forming the titanium oxide layeror the alumina/titania layer on the porous substrate can not onlyproduce a raw material for the hydroxide-ion conductive layeredcompound, but also serve as a seed for crystalline growth of thehydroxide-ion conductive layered compound and uniformly form thefunctional layer containing the hydroxide-ion conductive layeredcompound that is highly densified in the porous substrate. In addition,in Step (3), the presence of urea raises the pH value by generation ofammonia in the solution through the hydrolysis of urea, and gives thehydroxide-ion conductive layered compound (i.e., the LDH and/or theLDH-like compound) by formation of hydroxide with coexisting metal ions.Also, generation of carbon dioxide in hydrolysis gives the hydroxide-ionconductive layered compound of a carbonate anion type.

In particular, a composite material (that is, the LDH separator) inwhich the porous substrate is composed of a polymeric material and thefunctional layer is embedded over the porous substrate in the thicknessdirection is produced by applying the mixed sol of alumina and titaniato the substrate in Step (2) in such that the mixed sol permeates intoall or most area of the interior pores of the substrate. By this manner,most or substantially all pores inside the porous substrate can beembedded with the hydroxide-ion conductive layered compound. Examples ofpreferred application include dip coating and filtration coating.Particularly preferred is dip coating. The amount of the deposited mixedsol can be varied by adjusting the number of times of coating such asdip coating. The substrate coated with the mixed sol by, for example,dip coating may be dried and then subjected to Steps (3) and (4).

Secondary Zinc Batteries

The LDH separator of the present invention is preferably applied tosecondary zinc batteries. According to a preferred embodiment of thepresent invention, a secondary zinc battery comprising the LDH separatorare provided. A typical secondary zinc battery includes a positiveelectrode, a negative electrode, and an electrolytic solution, andisolates the positive electrode from the negative electrode with the LDHseparator therebetween. The secondary zinc battery of the presentinvention may be of any type that includes a zinc negative electrode andan electrolytic solution (typically, an aqueous alkali metal hydroxidesolution). Accordingly, examples of the secondary zinc battery includesecondary nickel-zinc batteries, secondary silver oxide-zinc batteries,secondary manganese oxide-zinc batteries, secondary zinc-air batteries,and various other secondary alkaline zinc batteries. For example, thesecondary zinc battery may preferably be a secondary nickel-zincbattery, the positive electrode of which contains nickel hydroxideand/or nickel oxyhydroxide. Alternatively, the secondary zinc batterymay be a secondary zinc-air battery, the positive electrode of which isan air electrode.

Solid-State Alkaline Fuel Cells

The LDH separator of the present invention can be applied to asolid-state alkaline fuel cell. The use of the LDH separator thatincludes the porous polymeric substrate having the pores plugged withthe hydroxide-ion conductive layered compound and is highly densified tohave a mean porosity of 0.03% to less than 1.0% makes it possible toprovide a solid-state alkaline fuel cell that can effectively prevent areduction in electromotive force caused by permeation of a fuel (forexample, by cross-over of methanol) into an air electrode: The hydroxideion conductivity of the LDH separator can effectively prevent permeationof a fuel, for example, methanol through the LDH separator. Thus,another preferred embodiment of the present invention provides asolid-state alkaline fuel cell including the LDH separator. A typicalsolid-state alkaline fuel cell includes an air electrode receivingoxygen, a fuel electrode receiving a liquid fuel and/or a gaseous fuel,and the LDH separator interposed between the fuel electrode and the airelectrode.

Other Batteries

The LDH separator of the present invention can be used not only innickel-zinc batteries or solid-state alkaline fuel cells but also in,for example, nickel-hydrogen batteries. In this case, the LDH separatorserves to block a nitride shuttle (movement of nitrate groups betweenelectrodes), which is a factor of the self-discharging in the battery.The LDH separator of the present invention can also be applied in, forexample, lithium batteries (batteries having a negative electrodecomposed of lithium metal), lithium ion batteries (batteries having anegative electrode composed of, for example, carbon), or lithium-airbatteries.

Examples

The invention will be further described in more detail by the followingExamples. The following procedures were used to evaluate the LDHseparator produced in these Examples.

Evaluation 1: Identification of LDH Separator

The crystalline phase of the LDH separator was measured with an X-raydiffractometer (RINT TTR III manufactured by Rigaku Corporation) at avoltage of 50 kV, a current of 300 mA, and a measuring range of 10° to70° to give an XRD profile. The resultant XRD profile was identifiedwith the diffraction peaks of LDH (hydrotalcite compound) described inJCPDS card NO.35-0964.

Evaluation 2: Measurement of Thickness

The thickness of each LDH separator was measured with a micrometer. Thethickness was measured at three points on the LDH separator. The meanvalue was calculated from these measurements and defined as thethickness of the LDH separator.

Evaluation 3: Measurement of Mean Porosity

A cross-sectional face of each LDH separator was polished with across-sectional polisher (CP). Two fields of images of thecross-sectional face of the LDH separator were captured at amagnification of 50,000 folds with a FE-SEM (ULTRA55 available from CarlZeiss). Based on the image data, the porosities of the two fields werecalculated with image inspection software (HDevelop available from MVTecSoftware). The average of the porosities was defined as the meanporosity.

Evaluation 4: Continuous Charge Test

A device 210 was assembled as shown in FIG. 2 and an accelerated testwas carried out to continuously grow zinc dendrites. Specifically, arectangular container 212 made of ABS resin was prepared, in which azinc electrode 214 a is separated by 0.5 cm from a copper electrode 214b to face each other. The zinc electrode 214 a is a metal zinc plate,and the copper electrode 214 b is a metal copper plate. In addition, aLDH separator structure including the LDH separator 216 was constructed,such that an epoxy resin-based adhesive was applied along the outerperiphery of the LDH separator, and the LDH separator was bonded to ajig made of ABS resin having an opening at the center. At this time, thebonded area between the jig and the LDH separator was sufficientlysealed with the adhesive to ensure liquid-tightness. The LDH separatorstructure was then disposed in the container 212 to isolate a firstsection 215 a including the zinc electrode 214 a from a second section215 b including the copper electrode 214 b, inhibiting liquidcommunication other than the area of the LDH separator 216. In thisconfiguration, three outer edges of the LDH separator structure (orthree outer edges of the jig made of ABS resin) were bonded to the innerwall of the container 212 with an epoxy resin adhesive to ensureliquid-tightness.

In other words, the bonded area between the separator structureincluding the LDH separator 216 and the container 212 was sealed toinhibit the liquid communication. 5.4 mol/L aqueous KOH solution as anaqueous alkaline solution 218 was poured into the first section 215 aand the second section 215 b along with ZnO powders equivalent tosaturated solubility. The zinc electrode 214 a and the copper electrode214 b were connected to a negative terminal and a positive terminal ofthe constant-current power supply, respectively, and a voltmeter wasalso connected in parallel with the constant-current power supply. Theliquid level of the aqueous alkaline solution 218 was determined belowthe height of the LDH separator structure (including the jig) such thatthe entire area of the LDH separator 216 in both the first section 215 aand the second section 215 b was immersed in the aqueous alkalinesolution 218. In the measurement device 210 having such a configuration,a constant current of 20 mA/cm² was continuously applied between thezinc electrode 214 a and the copper electrode 214 b for up to 200 hours.During application of the constant current, the voltage between the zincelectrode 214 a and the copper electrode 214 b was monitored with avoltmeter to check for short circuit caused by zinc dendrites (a sharpvoltage drop) between the zinc electrode 214 a and the copper electrode214 b. No short circuit for over 100 hours (or over 200 hours) wasdetermined as “(short circuit) not found”, and short circuit within lessthan 100 hours (or less than 200 hours) was determined as “(shortcircuit) found”.

Evaluation 5: Helium Permeability

A helium permeation test was conducted to evaluate the density of theLDH separator from the viewpoint of helium permeability. The heliumpermeability measurement system 310 shown in FIGS. 3A and 3B wasconstructed. The helium permeability measurement system 310 wasconfigured to supply helium gas from a gas cylinder filled with heliumgas to a sample holder 316 through the pressure gauge 312 and a flowmeter 314 (digital flow meter), and to discharge the gas by permeatingfrom one side to the other side of the LDH separator 318 held by thesample holder 316.

The sample holder 316 had a structure including a gas supply port 316 a,a sealed space 316 b and a gas discharge port 316 c, and was assembledas follows: An adhesive 322 was applied along the outer periphery of theLDH separator 318 and bonded to a jig 324 (made of ABS resin) having acentral opening. Gaskets or sealing members 326 a, 326 b made of butylrubber were disposed at the upper end and the lower end, respectively,of the jig 324, and then the outer sides of the members 326 a, 326 bwere held with supporting members 328 a, 328 b (made of PTFE) eachincluding a flange having an opening. Thus, the sealed space 316 b waspartitioned by the LDH separator 318, the jig 324, the sealing member326 a, and the supporting member 328 a. The supporting members 328 a and328 b were tightly fastened to each other with fastening means 330 withscrews not to cause leakage of helium gas from portions other than thegas discharge port 316 c. A gas supply pipe 334 was connected to the gassupply port 316 a of the sample holder 316 assembled as above through ajoint 332.

Helium gas was then supplied to the helium permeability measurementsystem 310 via the gas supply pipe 334, and the gas was permeatedthrough the LDH separator 318 held in the sample holder 316. A gassupply pressure and a flow rate were then monitored with a pressuregauge 312 and a flow meter 314. After permeation of helium gas for oneto thirty minutes, the helium permeability was calculated. The heliumpermeability was calculated from the expression of F/(P×S) where F(cm³/min) was the volume of permeated helium gas per unit time, P (atm)was the differential pressure applied to the LDH separator when heliumgas permeated through, and S (cm²) was the area of the membrane throughwhich helium gas permeates. The permeation rate F (cm³/min) of heliumgas was read directly from the flow meter 314. The gauge pressure readfrom the pressure gauge 312 was used for the differential pressure P.Helium gas was supplied such that the differential pressure P was withinthe range of 0.05 to 0.90 atm.

Evaluation 6: Measurement of Ionic Conductivity

The ionic conductivity of the LDH separator in the electrolytic solutionwas measured with an electrochemical measurement system shown in FIG. 4. A LDH separator sample S was sandwiched between two silicone gaskets440 having a thickness of 1 mm and assembled into a PTFE flange-typecell 442 having an inner diameter of 6 mm. Electrodes 446 made of #100nickel wire mesh were formed into a cylindrical shape having a diameterof 6 mm, and assembled into the cell 442, and the distance between theelectrodes was 2.2 mm. The cell 442 was filled with 5.4 M aqueous KOHsolution as an electrolytic solution 444. Using the electrochemicalmeasurement system (potentio-galvanostat frequency responsive analyzers1287A and 1255B, manufactured by Solartron), the sample was subjected tomeasurement under the conditions of a frequency range of 1 MHz to 0.1 Hzand an applied voltage of 10 mV, and the resistance of the LDH separatorsample S was determined from the intercept across a real number axis.The conductivity was calculated with the resistance, the thickness, andthe area of the LDH separator.

Example 1 (Comparative) (1) Preparation of Polymeric Porous Substrate

A commercially available polyethylene microporous membrane having aporosity of 50%, a mean pore size of 0.1 μm and a thickness of 20 μm asa polymeric porous substrate was cut out into a size of 2.0 cm×2.0 cm.

(2) Coating of Alumina/Titania Sol on Polymeric Porous Substrate

An amorphous alumina solution (Al-ML15, manufactured by Taki ChemicalCo., Ltd.) and a titanium oxide sol solution (M6, manufactured by TakiChemical Co., Ltd.) were mixed at Ti/AI molar ratio of 2 to yield amixed sol. The mixed sol was applied onto the substrate prepared inProcess (1) by dip coating. In dip coating, the substrate was immersedin 100 mL of the mixed sol, pulled up vertically and dried in a dryer at90° C. for five minutes.

(3) Preparation of Aqueous Raw Material Solution

Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, manufactured by KantoChemical Co., Inc.), and urea ((NH₂)₂CO, manufactured by Sigma-AldrichCorporation) were provided as raw materials. Nickel nitrate hexahydratewas weighed to be 0.015 mol/L and placed in a beaker. Ion-exchangedwater was added into a total volume of 75 mL. After stirring thesolution, the urea weighed at a urea/NO₃ ⁻ molar ratio of 16 was added,and further stirred to give an aqueous raw material solution.

(4) Formation of Membrane by Hydrothermal Treatment

The aqueous raw material solution and the dip-coated substrate wereencapsulated into a Teflon™ autoclave (the internal volume: 100 mL,covered with stainless steel jacket). The substrate was horizontallyfixed away from the bottom of the Teflon™ autoclave such that thesolution was in contact with the two surfaces of the substrate. A LDHwas then formed on the surface and the interior of the substrate by ahydrothermal treatment at a temperature of 120° C. for 24 hour. After apredetermined period, the substrate was removed from the autoclave,washed with ion-exchanged water, and dried at 70° C. for ten hours toform the LDH in the pores of porous substrate and give a compositematerial containing the LDH.

(5) Densification by Roll Pressing

The composite material containing the above LDH is sandwiched between apair of PET films (Lumirror™ manufactured by Toray Industries, Inc., athickness of 40 μm), and then roll-pressed at a rotation rate of 3 mm/s,at a roller temperature of 110° C., and with a gap between rollers of 60μm to give a LDH separator.

(6) Results of Evaluation

The resultant LDH separator was evaluated in accordance with Evaluations1 to 6. As a result of Evaluation 1, this LDH separator was identifiedas LDH (hydrotalcite compound). The results of Evaluations 2 to 6 are asshown in Table 1. As shown in Table 1, zinc dendrites did not causeshort circuiting after continuous charge up to 100 hours, but causedshort circuiting after continuous charge for less than 200 hours inEvaluation 4.

Examples 2 to 7

Each LDH separator was produced and evaluated as in Example 1 exceptthat the roller heating temperature was varied to the values shown inTable 1 in the densification by roll pressing in Process (5). As aresult of Evaluation 1, the LDH separators in these Examples wereidentified as LDH (hydrotalcite compound). Table 1 shows the results ofEvaluations 2 to 6. As shown in Table 1, zinc dendrites did not causeshort circuiting even after continuous charge for 200 hours or more inExamples 2 to 7. FIG. 5 shows a cross-sectional FE-SEM image, capturedin Evaluation 3, of the LDH separator according to Example 4.

Example 8 (Comparative)

The LDH separator was produced and evaluated as in Example 1 except thatthe densification by roll pressing in Process (5) was not carried out.The results of Evaluation 1 indicated that the LDH separator of thisExample was identified as LDH (hydrotalcite compound). Table 1 shows theresults of Evaluations 2 to 6. As shown in Table 1, zinc dendritescaused short circuiting after continuous charge for less than 100 hoursin Evaluation 4. FIG. 6 shows a cross-sectional FE-SEM image, capturedin Evaluation 3, of the LDH separator.

Examples 9 to 11

A LDH separator was produced and evaluated as in Example 1 except forthe following conditions a) to c).

a) Magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O, manufactured by KantoChemical Co., Ltd.) was used instead of the nickel nitrate hexahydratein Process (3), weighed to be 0.03 mol/L, and placed in a beaker.Ion-exchanged water was added into a total volume of 75 mL. Afterstirring the resultant solution, the urea weighed at a urea/NO₃ molarratio of 8 was added, and further stirred to give an aqueous rawmaterial solution.b) The hydrothermal temperature in Process (4) was 90° C.c) The roller heating temperature was varied to the values shown inTable 1 in the densification by roll pressing in Process (5).

As a result of Evaluation 1, this LDH separator was identified as LDH(hydrotalcite compound). The results of Evaluations 2 to 6 are shown inTable 1. As shown in Table 1, zinc dendrites did not cause shortcircuiting even after continuous charge for 200 hours or more inExamples 9 to 11.

TABLE 1 Evaluations of LDH separator Roll Continuous charge testpressing Short circuiting Short circuiting Composition Roll Mean HeliumIon over continuous over continuous of LDH Temp. Thickness porositypermeability conductivity charging time charging time separator (° C.)(μm) (%) (cm/min · atm) (mS/cm) up to 100 hours up to 200 hours Ex. 1*Ni—Al,Ti 100 13 1.0 0.0 3.0 None Found Ex. 2  Ni—Al,Ti 110 13 0.9 0.02.8 None None Ex. 3  Ni—Al,Ti 120 13 0.8 0.0 2.8 None None Ex. 4 Ni—Al,Ti 130 12 0.5 0.0 2.6 None None Ex. 5  Ni—Al,Ti 150 12 0.1 0.0 2.3None None Ex. 6  Ni—Al,Ti 170 11  0.05 0.0 2.2 None None Ex. 7  Ni—Al,Ti180 11  0.03 0.0 2.0 None None Ex. 8* Ni—Al,Ti — 16 20   100    3.1Found — Ex. 9  Mg—Al,Ti 110 13 0.9 0.0 2.7 None None Ex. 10 Mg—Al,Ti 15012 0.2 0.0 2.3 None None Ex. 11 Mg—Al,Ti 180 11  0.03 0.0 2.1 None None*indicates Comparative Example.

What is claimed is:
 1. A layered double hydroxide (LDH) separatorcomprising a porous substrate made of a polymeric material; and ahydroxide-ion conductive layered compound being a LDH and/or a LDH-likecompound with which pores of the porous substrate are plugged, whereinthe LDH separator has a mean porosity of 0.03% to less than 1.0%.
 2. TheLDH separator according to claim 1, wherein the mean porosity is 0.05%to 0.9%.
 3. The LDH separator according to claim 1, wherein thehydroxide-ion conductive layered compound is incorporated over theentire thickness of the porous substrate.
 4. The LDH separator accordingto claim 1, having a helium permeability per unit area of 3.0 cm/atm·minor less.
 5. The LDH separator according to claim 1, having an ionicconductivity of 2.0 mS/cm or more.
 6. The LDH separator according toclaim 1, wherein the polymeric material is selected from the groupconsisting of polystyrene, poly(ether sulfone), polypropylene, epoxyresin, poly(phenylene sulfide), fluorocarbon resin, cellulose, nylon,and polyethylene.
 7. The LDH separator according to claim 1, consistingof the porous substrate and the hydroxide-ion conductive layeredcompound.
 8. A secondary zinc battery comprising the LDH separatoraccording to claim
 1. 9. A solid-state alkaline fuel cell comprising theLDH separator according to claim 1.